In nuclear medicine, positron emission tomography is an imaging technique requiring radioisotopes imaging positrons or molecules marked by those same radioisotopes. 18F is one of the most commonly used radioisotopes from among others such as 13N, 15O or 11C. 18F has a half-life time of 109.6 min. and can thus be conveyed toward sites other than its production site.
18F is most often produced in its ionic form and obtained by accelerated proton bombardment on an irradiation cell comprising 18O-enriched water. Many irradiation cells have been developed that all have the same purpose of producing 18F in a shorter period of time with a better yield. Generally, the production of radioisotopes comprises a proton accelerator and an irradiation cell. This irradiation cell comprises a cavity, inside which the radioisotope precursor in liquid form is included.
Generally, the energy from the proton beam directed on the irradiation cell is approximately from several MeV to approximately 20 MeV. Such a beam energy causes heating of the irradiation cell as well as vaporization of the radioisotope precursor, thereby decreasing the stopping power of that precursor and therefore the radioisotope production yield. A device cooling the target must therefore also be used so as to try to keep the radioisotope precursor in liquid form, or at most in an intermediate state between liquid and vapor. Furthermore, in the case of 18F production, due to the particularly high cost of the precursor, 18O-enriched water, only a very small volume of that precursor, at most several milliliters, can be placed in the irradiation cell. Consequently, the issue of heat dissipation produced by the irradiation of the target material on such a small volume is a major problem to be overcome. Typically, the power to be dissipated for an energy beam of 18 MeV with an intensity from 50 to 100 μA is between 900 W and 2,700 W, over a radioisotope precursor volume generally comprised between 0.2 and 5 ml, for irradiation times from several minutes to several hours.
Document U.S. Pat. No. 5,917,874 describes a radioisotope production device comprising an irradiation cell closed by a metal foil and comprising a fluid comprising a radioisotope precursor or target fluid. The depth of the cavity of the irradiation cell with respect to the axis of the beam is relatively small so as to irradiate substantially the entire target fluid sample. In one preferred usage example of that document, the depth of the cavity of the irradiation cell is 1.7 mm, so as to have an optimal working cross-section to produce radioisotopes. The energy from the particle beam irradiating the target fluid is approximately 8 MeV, which requires a thin enough metal foil to limit energy losses of the beam when the latter passes through the foil. In the cited document, the foil has a thickness of approximately 6 microns and is held by a perforated grid so as to bear the increasing internal pressure in the irradiation cell during irradiation. The radioisotope production device also comprises means for cooling the irradiation cell. The irradiation cell can be inserted into a cooling housing in which a stream of water circulates. The irradiation cell also comprises a solid column made from a material with a high heat conductivity and situated on the rear side of the irradiation cell, across from the foil, so as to evacuate the heat produced in the cavity. The inside of the cooling housing is cylindrical and comprises a conduit situated across from the apex of said cone and designed to project a turbulent flow of cold water on the cone. The cone also has fins spaced radially around the surface thereof, so as to improve the evacuation of the heat. Such a device only enables the irradiation of small volumes of 18O-enriched water, and does not have the means making it possible to effectively cool the metal foil, which can be problematic in terms of the sealing of the irradiation cell. Furthermore, the perforated grid is not completely transparent to the beam and prevents part of the beam from penetrating inside the cavity. Part of the perforated grid or the metal foil therefore absorbs part of the beam, which causes heating of the metal foil. The metal foil being relatively thin and being the most heated and least well-cooled part, it is relatively fragile. Furthermore, the seals situated between the latter and the body are damaged during use and said cavity loses sealing.
Document BE 1011263 describes a radioisotope production device comprising an irradiation cell with a cavity comprising a hemispherical part, closed by a foil. The irradiation cell receives a fluid comprising a radioisotope precursor also called “target fluid.” The walls of this cavity are made from a heat-conducting metal material and with a thin enough thickness to dissipate the heat from the inside of the cavity. An element called a “diffuser” surrounds the outer walls of said cavity, creating a channel in which a coolant circulates. Nevertheless, the irradiation cell must have a minimal depth so as to sufficiently irradiate target fluid without beam losses in the body of the irradiation cell. In order to increase the depth of the cavity without significantly increasing the volume of the cell so as to minimize the quantity of 18O inside the cell, an irradiation cell as described in document WO 2005081263 has been produced. This irradiation cell comprises a first cylindrical part and a second hemispherical part situated on the side opposite a foil closing the cavity. A first drawback of this type of device is that when the irradiation cell is irradiated, a large portion of the fluid comprised in the cavity vaporizes, leaving only a thin net of water on the lower wall of the cavity. The particle beam passing through a low-density volume, the likelihoods of 18O(p,n)18F nuclear reactions are decreased. Furthermore, the walls of the cavity being relatively thin and undergoing significant heating, said cavity collapses after several uses, which positions part of the liquid, which is already not very irradiated, outside the beam and causes a drop in yield.
Document US 20050084055 describes a radioisotope production device comprising an irradiation cell comprising a target fluid. The irradiation cell comprises a cavity closed by a foil. The cavity comprises a face opposite said foil and called “rear wall,” as well as an upper wall situated at the top of the foil and the rear wall. The rear wall is inclined such that the part of the rear wall proximal to the upper wall is further from the foil than the part of the rear wall distal to the upper wall. The device also comprises a cooling system comprising a vertical conduit 502 through which a coolant arrives. The vertical conduit 502 is connected to a conduit 504 and adjacent to the rear wall, which in turn is connected to a conduit 506 adjacent the upper wall. In this device, the lower wall separating said surface from the part of the rear wall distal to the upper wall is not cooled. Furthermore, the cooled walls are only cooled by a conduit in contact with part of the walls. Lastly, the fluid present in the cavity condenses on the cooled upper wall through a liquid having been heated after having passed through the conduit 504 and adjacent to the rear wall. The cooling of the fluid comprised in the cavity is therefore not optimal and must be improved so as to have more condensed liquid across from the beam so as to increase the probabilities of nuclear reactions.
With the aim of reducing the mechanical stresses on the foil due to the increase in pressure in the cavity during irradiation, document U.S. Pat. No. 6,586,747 describes a radioisotope production device comprising an irradiation cell comprising a cavity closed by a foil that is inclined relative to the axis of the beam. In this way, the power of the beam is distributed over a larger area. Nevertheless, in this device, with the increase in the area of the foil exposed to the beam, the power of the beam dissipated in the foil nevertheless remains high, which causes overall heating of the foil and an increase in the inner pressure in the cavity.
Document US 20060062342 aims to resolve the problem of pressure stresses on the foil by introducing a pressurized chamber adjacent to the foil of the irradiation cell, such that the pressure exerted on the foil on the side of the pressurization chamber opposes the pressure exerted on the same foil on the side of the irradiation cell. The incline or perpendicular position relative to the beam of the target chamber foil should make it possible to force the target fluid under the apex of the foil. Nevertheless, the device does not comprise a system for cooling the foil, and the addition of a pressurized chamber and therefore additional foil in the passage of the beam causes power losses of the beam. The foil being poorly cooled, it is difficult to force the fluid against the apex of said foil.
The document FIROUZBAKHT M. L. et al, “Mechanism of nitrogen-13-labeled ammonia formation in a cryogenic water target—Target design, products and operating parameters,” Nuclear Medicine and Biology, ELSEVIER, N.Y. vol. 26, no. 4, May 1, 1999, pages 437-441, describes a target with a conical cavity cooled by a cryogenic liquid. The foil forming the irradiation window is separated from the conical cavity by an annular channel that serves to collect, through gravity, at the lowest level, the liquid the condenses on the walls of the cavity.
The document FIROUZBAKHT M. L. et al, “Cryogenic target design considerations for the production of [18F]fluoride from enriched [18O] carbon dioxide,” Nuclear Medicine and Biology, ELSEVIER, N.Y., vol. 26, no. 7, Oct. 1, 1999, pages 749-753, also describes targets cooled by a cryogenic liquid. The cavity of the target of FIG. 1 comprises a cylindrical part extended by a conical part. The metal foil forming the irradiation window closes the cylindrical part of the cavity. A target of the same type is also described in the document by T. KAKAVAND et al., “Computer simulation techniques to design Xenon-124 solid target for iodine-123 production,” IRANIAN JOURNAL RADIATION RESEARCH, vol. 5, no. 4, 2008, pages 207-212.
It will be noted that cryogenic cooling targets cause fewer problems regarding cooling of the cell and the metal foil forming the irradiation window. They do, however, require finding gaskets that withstand cryogenic temperatures and, at the same time, have a sufficient lifetime when exposed to intense irradiation.
In order to increase the radioisotope production yields, it is necessary to provide a radioisotope production device that does not comprise the drawbacks of the prior art.
In particular, it is necessary to provide effective means for cooling the window closing the target cavity, particularly when working with a non-cryogenic coolant.
It is also necessary to improve the cooling device for the walls of the target cavity.
Other advantages and properties of the device according to the invention will be shown in light of the following description.